This post has been updated with a more accurate description of the use of the chant in question, and a video posted yesterday of a live recording of it at the Ukrainian Greek-Catholic Cathedral of St George in Lviv. The new video begins with one of the best Alleluias of the Slavonic chant repertoire.
Behold the Bridegroom cometh in the midst of the night, and blessed is the servant whom He shall find watching; and again, unworthy is he whom He shall find heedless. Take care, therefore, oh my soul, lest thou be borne down down with sleep, lest thou be given up to death, and be shut out of the kingdom; but rouse thyself, crying, Holy, Holy, Holy are Thou O God. * Through the Mother of God, have mercy on us!
Се Женихъ грѧдетъ въ полунощи, и блаженъ рабъ, егоже обрѧщетъ бдѧща: недостоинъ же паки, егоже обрѧщетъ унывающа. блюди убо душе моѧ, не сномъ отѧготисѧ, да не смерти предана будеши, и Царствиѧ вне затворишисѧ, но воспрѧни зовущи: Свѧтъ, Свѧтъ, Свѧтъ єси Боже, Богородицею помилуй нас.
Matins are traditionally anticipated to the evening of the day before, so that the first of the Bridegroom Matins, that of Holy Monday, is celebrated on the evening of Palm Sunday, the second, that of Holy Tuesday, on the evening of Holy Monday, and the third, that of Holy Wednesday, on the evening of Holy Tuesday. According to a Greek Holy Week book which I have, the troparion is sung three times in a row, but the final words “through the Mother of God, have mercy on us!” as given above are only sung the third time. On Holy Monday, the first two times end with the words “by the protection of the Bodiless Ones”, on Holy Tuesday, “by the prayers of the Forerunner”, and on Holy Wednesday, “by the power of the Cross.”
Here is a another very beautiful version in Arabic.
From the Ukrainian Greek-Catholic Cathedral of St George in Lviv: